For the past year, the former wife of Rolling Stone Mick Jagger has been shedding her reputation as a glamorous, frivolous international socialite and has been waging a personal campaign in the United States and Latin America on behalf of Salvadoran refugees.

It's been a year of lending her celebrity to fund-raising efforts and speaking out against the violation of refugees' human rights. Her new role has taken her light-years away from her old world of New York's Studio 54 disco, but the former model can still be seen at some of New York's and London's most exclusive gatherings.

Ms. Jagger was in Washington recently to speak at a fund-raiser for the Salvadoran Humanitarian Aid, Research, and Education Foundation, a nonpartisan, tax-exempt organization that channels refugee aid through the Roman Catholic Church in Mexico and El Salvador.

The group estimates there are 125,000 Salvadoran refugees in El Salvador itself and 250,000 more scattered throughout Honduras, Costa Rica, Mexico, Panama, and Belize. Most had hoped to escape the Salvadoran civil war by fleeing across the border to Honduras.

The World Council of Churches, Amnesty International, and the United Nations have reported that conditions in the refugee camps are grim: They are overcrowded, have poor shelter and sanitation, and provide inadequate food, clothing, and medical care.

Over a breakfast of toasted English muffins and tea at her Washington hotel, Ms. Jagger explained that she feels compelled to devote most of her time to refugee work in part because her presence draws attention to an issue that might otherwise get short shrift.

''You feel that you have to do it because maybe you have a privileged position,'' she said. ''Maybe you can accomplish just a little bit more than other people only because of something that is not really due to you, but it really doesn't matter if it is used in a good cause.''

She says that her transformation from jet setter to crusader was a slow and painful evolution - an odyssey that began when she was a child in her native Nicaragua. The daughter of a wealthy Managuan businessman, she led a privileged existence in a country where wealth was held by only a few immensely rich families and most of the population lived at a subsistence level.

Her Spanish accent still apparent in her precise English, Ms. Jagger explains that her parents divorced when she was 10 and ''my mother had to go and work. I suddenly had the other side of the coin in the sense that before, everything was due to us, we thought.

''Suddenly, things became much more difficult for us. My mother just worked hard to be able to keep us in the same school, to be able to maintain the same standard that we had in the past. . . . But I was too young to say that I was fully aware of the realities.''

At 16, Bianca Perez Morena de Macais left Nicaragua to study political science in Paris on a scholarship from the French government. There she met Mick Jagger. They were married in St. Tropez in 1971.

Two years later, an earthquake leveled most of Managua, and Ms. Jagger caught the first available flight to the Nicaraguan capital. There she faced for the first time not only enormous suffering but also the corruption and indifference of former dictator Anastasio Somoza.

''You found kids dying in the street, and you found people wouldn't be fed unless they had a little red flag that said they had voted for Somoza,'' she recalled. ''Most of the international aid went directly to Somoza. . . . I was suddenly confronted with something - it was as if I had suddenly opened my eyes to something I never knew before.''

She returned to Europe and persuaded the Rolling Stones to stage a benefit concert, which raised $280,000 for the earthquake victims. Returning to Managua to distribute the funds, she found that ''first, we had to have a big battle with Somoza to allow us to give the money to an independent organization so that it wouldn't go to him.''

During the 1979 revolution that toppled Somoza, Ms. Jagger went to Managua several times to see if her mother, who ran a diner attached to her house, was all right and - occasionally dodging sniper fire - to determine what could be done to help her people. She helped the British Red Cross raise funds for war victims.

Visiting a country at war ''gives you a different perspective on life,'' she says. ''You're totally indifferent, totally insensitive, if you're not forced to see things completely differently from that moment on.''

About two years ago, leftist guerrillas began to heat up the long-running civil war in El Salvador, and an incident last year set Ms. Jagger on her course as a refugee advocate.

During a visit to Managua, she met a group of journalists who had just come from refugee camps in Honduras. ''They were shocked by the conditions,'' she recalled. ''I decided to go there in September 1981.

''The refugees were in a camp that was in the bottom of a ravine. The conditions were precarious, and a lot of children had died from malnutrition and disease. They were always in fear. They were persecuted by the Salvadoran Army, who would cross the border into the refugee camp.''

She also contends they were harassed and ''persecuted'' by the Honduran Army, which labeled the refugees ''subversives.''

Ms. Jagger took her story to Washington, met with several members of Congress , and began organizing a congressional delegation to visit the camps. Before she could assemble the fact-finding team, ''we got a phone call saying it was imperative we come right away.'' So with a small contingent, including an aide to Rep. Ron Dellums (D) of California and Jim Stephens, a scholar specializing in Central America, she flew into La Virtud, a camp on the Honduran side of the border housing about 8,000 Salvadorans, last Nov. 16.

Shortly after their arrival, a patrol of uniformed Salvadoran soldiers entered the camp compound. Moments later, ''a relief worker came in screaming, 'They've taken the refugees and they're going to kill them!'

''Ms. Jagger's group jumped in a jeep with the worker and rushed to a bridge at the entrance to the camp, where they saw a second group of armed men. There were 12 to 15 men wearing civilian clothes in this group, their faces covered with bandannas. They carried US-made M-16 rifles and ''sophisticated radio equipment,'' Ms. Jagger recalled.

With them were 15 to 18 refugees with their hands tied behind their backs. A pregnant woman was one of the prisoners, and one of the abductors ''kept kicking her, and she kept falling,'' Ms. Jagger said.

''We started screaming, and one of the refugees - because of this screaming - managed to escape. This gave us the idea that maybe we could do something, so we started to follow along this river. We were screaming and all the kids from the refugee camp were running along screaming, 'Mommy, mommy, daddy, daddy!' ''

After about 20 minutes, some of the refugees broke and ran toward the international group. ''The Salvadorans turned around, and at that point, it was a moment where it was either they kill everybody, including us, or - something happened. We don't know what happened, but the Salvadorans turned around and continued on toward El Salvador.

''So we came back to the refugee camp, and everybody was kneeling and thanking God for being able to be alive.''

Some of the refugees in the camp said they recognized some of the abductors as being members of the Salvadoran paramilitary forces, similar to a national guard unit. Ms. Jagger says a Honduran military contingent near the camp refused to investigate the incident and that the Honduran government has frequently cooperated with Salvadoran forces making incursions into Honduras.

A few days after the incident, she returned to Washington to testify on the plight of the refugees before the House Inter-American Affairs Subcommittee.

Since then, most of the 30,000 refugees huddled along the Salvador-Honduras border have been relocated to camps farther in the interior of Honduras. Some of the abductions and killings by Salvadoran forces may have stopped, she says, but by forcibly moving them from the border, the resettlement effort has plunged the refugees into despair.

''They are totally at the mercy of the Honduran military,'' she complained. ''They are not even allowed to go out and cut some wood that is necessary for them to be able to eat and cook.''

At La Virtud, the refugees were ''very industrious,'' she recalled, but at the new camps in the interior, ''they do nothing. They feel they have lost everything, that they are forgotten.''

Her greatest concern for the future is that the Reagan administration's plans to increase military aid to Central America could inadvertently spur guerrilla movements throughout Central America - and create even more refugees.

Honduras, which trails only El Salvador in the amount of US military assistance to Central America, ''was one of the few countries in the region with a hope that a democracy could be established,'' she declared. But the massive infusion of US aid has swung the real power in the country away from the newly elected civilian government and to the military, she argues.

''Suddenly we are seeing all the signs of what we saw in the past in El Salvador, and it would be very sad if Honduras was to play the role that Cambodia played in the Southeast Asian war.''

She dismisses President Reagan's fears that communist countries such as Cuba are fomenting revolution in Latin America. ''To simplify this situation and to say it is a threat coming from the outside is too easy,'' she contended. ''You have to go to the root of things.

''Any country where you have a long history of injustice and of oppression and such a difference in class, you will have at one point or another upheaval. The people will decide that it is time to find something better. . . .

''The majority of them can't read or write. They don't know what communism means. They do know what it means to be hungry. They know what it means to be homeless. . . . They know what it means not to have a job. And they know what it means to be under an oppressive government.''